I have a 99 ML 320 with a machine gun lock on the front passenger door. My local dealer wanted $267 (plus tax) for replacement. I Googled the part number (163-720-28-35) and found it for about $180 delivered from factoryoemparts.com
163-720-28-35 Mercedes Benz LOCK
Just ordered yesterday so don't know how well they'll do as a supplier or how difficult the repair is.